1. Field of the Invention
The present invention relates to an exposing apparatus which is used in the lithography process in manufacturing of a semiconductor integrated circuit and, more particularly, to a stage apparatus having a leveling function for inclining a photo sensitive substrate and making a predetermined exposure reference surface, for instance, a projection image plane of a mask pattern projected by a projection optical system accurately coincide with the surface of the photo sensitive substrate.
2. Related Background Art
In the lithography process, a reduction projection type exposing apparatus of the step and repeat system, what is called, a stepper, plays a central role. In a stepper, it is necessary to raise the resolution limit of the projection lens in correspondence to a line width of a pattern of a semiconductor device which is formed on the submicron order. The need for a projection lens which has both a large numerical aperture (N.A.) and a wide exposure field has increased substantially in recent years. However, a focal depth of the projection lens of a large N.A. and a wide exposure field is inevitably shallow. Therefore, even when a predetermined exposure reference surface accurately coincides with the surface of an arbitrary shot area on the wafer at the shot center (focus matching), if the surface of a part of the shot area is inclined from a predetermined exposure reference surface such as a projection image plane of the mask pattern, it is difficult to always execute accurate focus matching in the whole surface in the exposure field. Regarding this problem, a stepper having a leveling mechanism comprising an inclination detecting system (horizontal position detecting system) is disclosed in U.S. Pat. No. 4,558,949 and a stage apparatus (wafer stage) is disclosed in U.S. Pat. No. 4,770,531, wherein an inclination of the shot area from a predetermined exposure reference surface is detected by using the horizontal position detecting system. In such a stepper, a predetermined plurality of points (for instance, three operating points) are driven to incline a leveling stage forming part of the wafer stage and an angle of inclination of the leveling stage is controlled, thereby making the surface of the shot area accurately coincide with the exposure reference surface. In the initial adjustment of the stepper, a predetermined neutral state exists (for example, a state in which each operating point is located at the center of a movement stroke in the Z direction along the optical axis of the projection lens). Adjustment is executed in a manner such that a virtual rotation center point of the leveling stage serving as an apparent reference point of the inclining operation is located in the exposure reference surface, which includes the length measuring axis (center of the laser light flux) of a laser interferometer used to detect stage positions. The exposure reference surface coincides with the surface of a reference wafer which is mounted on the leveling stage and with the projection image plane (image forming surface) of the mask pattern projected by the projection lens. By making the surface of the shot area accurately coincide with the exposure reference surface for every shot, using the leveling mechanism, a circuit pattern of a mask or a reticle (hereinafter, simply referred to as a "reticle") is exposed on the wafer at high resolution and without focal deviation in the exposure field.
However, in the stepper having such a leveling mechanism, it is difficult to ensure that the virtual rotation center point of the leveling stage is accurately located in a surface such as an exposure reference surface or the like in the initial adjustment, from the viewpoint of accuracy in manufacturing of the leveling stage or the like. There is also a problem in that it takes a long time for such adjustment. Further, in the case of performing leveling by inclining the leveling stage by an angle .theta. in a state in which a deviation in the Z direction remains between the rotation center point and the exposure reference surface (that is, the wafer surface), a lateral deviation, what is called a sine error .DELTA.Se (.DELTA.Se=.nu..sin.theta.) due to a deviation amount .theta. in the Z direction occurs. Thus, the positioning accuracy (about 0.03 .mu.m) which is required for the wafer stage of such a stepper cannot be satisfied. On the other hand, there is also a problem in that in order to correct the lateral deviation amount, after leveling is executed for every shot, the fine alignment between the projection image of the reticle pattern and the circuit pattern on the wafer must be again executed, so that throughput deteriorates.